Flywheels are used to store energy in the form of rotational energy. Suitable coupling means (motor-generator units) can be used to store electric energy in these energy storage units in the form of rotational energy and, whenever necessary, this energy can be converted back into electric energy and can be transmitted to a consumer. For the most part, such flywheel energy storage units have a massive rotor which, depending on its rotational speed and mass, stores a certain amount of energy. The storage capacity of such an energy storage unit is limited by its mass and by its maximum rotational speed. In this context, the rotor is usually connected by suitable connection means to a bearing shaft that holds the rotor. The bearing shaft is mounted in appropriate bearings so that it can rotate. Here, the bearing elements and drive elements are preferably located on the axis of rotation of the rotor. In this context, the connection means have the function of creating a connection and an attachment between the external flywheel body (rotational body) and the carrying axle (shaft) as well as to ensure the transmission of the torsional moment from the drive shaft to the flywheel body (rotational body), and vice versa. The rotational bodies have to exhibit good-quality manufacturing, high strength and great stiffness under flexural load in order to achieve a reliable and trouble-free operation of the rotors.
Up until now, rotors with rotational bodies are generally configured as disk-shaped, very thick-walled components. The wall thickness of the rotational bodies is in the same order of magnitude as the diameter, whereas the length along the axis of rotation is smaller than or similar to the diameter. In this context, when carbon fiber-reinforced plastics (so-called CFRP materials) are used as the material for the rotational body, the flywheel disks usually consist of fibers oriented virtually in the circumferential direction.
Since the energy storage capacity exhibits a quadratic dependence on the rotational speed of the mass, this concept entails a few drawbacks. The fibers, which are situated relatively far towards the inside (near the axis of rotation), have a considerably lower rotational speed and therefore can hardly store energy. Moreover, this structure gives rise to large radial tensile stresses that can damage the rotors. This can only be avoided by means of complex design measures such as manufacturing several sub-rings and pressing them together to form a rotational body. Moreover, due to the thickness of the walls, the production is extremely difficult and laborious so that this always creates problems, for instance, with fiber kinks, the fiber volume content, waviness and delamination.
Nowadays, there is a widespread move towards regenerative energy sources, especially in view of their greater environmental friendliness in comparison to conventional energy generation systems. Since, as a rule, these regenerative energy sources do not deliver their energy continuously, there is a great need for energy storage units. For this reason, it is desirable for the energy content of such a flywheel energy storage unit to be raised even further and for the costs for their production to be lowered.